When an observer is looking at a real scene, a first two-dimensional (2D) image is seen by the left eye of the observer and a second 2D image is seen by the right eye of the observer. Only the brain fuses this pair of 2D images into a three-dimensional (3D) scene. By replicating such a perception of different views on a scene by means of a display, it is possible to fool the brain into perceiving a 3D scene as if it was actually receiving images of a real scene and not of the screen of a display.
For producing a 3D effect, stereoscopic and multi-view displays thus send a different two-dimensional (2D) image of a scene to the left eye and to the right eye of an observer. The 2D images comprise small horizontal differences in the location of points in the scene. This allows the brain to fuse the two 2D images into a single perceived 3D scene, in which the objects have a perceived depth which is related to the disparity between the 2D images seen by the eyes.
A 3D presentation by a display, however, may comprise inconsistencies and artefacts, which are not existent in real scenes. Such inconsistencies and artefacts cause strain on the brain and may result in fatigue, eye strain, headaches and fusion problems.
The three most significant causes of eye strain and fusion problems with stereoscopic displays arise from accommodation-convergence mismatch, misalignment and reverse half-occlusions. With stereoscopic displays, the eyes focus on the display but converge to a distance out of the display, which causes a difference between accommodation and convergence. Such an accommodation-convergence mismatch can be controlled by controlling the disparity in the content. Misalignment between the views in a pair of 2D images cause a range of effects ranging from perceived depth space warping to fusion problems and should be corrected in a suitable manner. The present document focuses on the third problem of reverse half-occlusions.
Occlusions occur naturally in real scenes, in which a near object blocks an object that is farther away from the observer. For example, when the observer is looking at the right edge of a near object, the object partially blocks the view to the background for the left eye, while the right eye can see around the object seeing some of the background that is blocked for the left eye. The brain uses this discrepancy as a depth indicator indicating that the background object continues behind the front object.
For illustration, FIG. 1a schematic presents a situation, in which a first object 111 is placed in front of a second object 112 from the point of view of an observer. FIG. 1b presents in addition on the left hand side the 2D image of the two objects 111, 112 as perceived by the left eye of the observer, and on the right hand side the 2D image of the two objects 111, 112 as perceived by the right eye of the observer.
The brain uses a number of depth indicators to create a perceived 3D scene from the two 2D images coming to the left and right retina. The most important depth indicator is horizontal disparity, which means that the left eye and the right eye perceive the visible edges of the objects of a scene at horizontally shifted locations. The simple fact that object 111 blocks the view to object 112 and that there is an area of object 112 that can only be seen by the right eye while being occluded for the left eye gives the brain in addition an indication that object 111 must be in front of object 112.
FIG. 2a schematically presents a similar situation, in which an observer looks at an object 212 through a window 200 in a wall 201, the object 212 being partially occluded by the wall 201. FIG. 2b presents on the left hand side the 2D image of the window 200 and the object 212 as perceived by the left eye of the observer, and on the right hand side the 2D image of the window 200 and the object 212 as perceived by the right eye of the observer.
The wall 201 blocking the view to the object 212 is an indicator that the window 200 is in front of the object 212. In addition, more of the object 212 is visible for the right eye than for the left eye, which provides a further indication that the object 212 is located behind the window 200.
A corresponding situation occurs in a presentation on a stereoscopic or multi-view screen, if a virtual object is located in the rear half of the virtual space, that is, behind the screen. For the illustration of such a situation, the window 200 of FIGS. 2a and 2b can be viewed as a screen of a stereoscopic display and object 212 can be viewed as a virtual object. In this case, the convergence, disparity and occlusion information all agree with each other and so the brain perceives the virtual 3D object 212 at a larger distance than the screen 200.
In the real world, objects can only occlude other objects that are located behind them. An object cannot occlude another object that is located in front of it. This is illustrated in FIGS. 3a and 3b. 
FIG. 3a schematically presents a situation, in which an observer looks at a real object 311 in front of a window 300 in a wall 301. FIG. 3b presents on the left hand side the 2D image of the window 300 and the object 311 as perceived by the left eye of the observer, and on the right hand side the 2D image of the window 300 and the object 311 as perceived by the right eye of the observer.
Since the object 311 is located in front of the window 300, it blocks out the view to a part of the window 300 and of the wall 301.
Moreover, the eyes converge on one object, for instance object 311, causing zero retinal disparity between the eyes at the point of convergence where the object falls in the middle of both eyes. In other areas of the view, another object, for instance the wall 301, is noticed at a different depth. The difference in depth is picked up by the retinal disparity between the edges detected for the second object. In the presented situation, the disparity gives an accurate measure of how far the object 311 is in front of the window 301. The eye muscle tension gives in addition an overall global depth estimate.
The resulting occlusion and depth information all match with what the brain is used to.
With stereoscopic or multi-view displays, in contrast, a problem of reverse half-occlusions may occur, whenever an object that is perceived to be located in front of the display touches the edge of the screen. The problem of reverse half-occlusions is illustrated in FIGS. 4a and 4b. 
FIG. 4a schematically presents a situation, in which an observer looks at a virtual object 411 in front of a screen 400 of a stereoscopic or multi-view display. The virtual object 411 is constructed in the brain of an observer from an object 411a in a left image presented on the screen 400 and from an object 411b in a right image presented on the screen 400. Object 411a and 411b are the same, but horizontally shifted against each other so that object 411b is cut off to a larger extent by the left edge of the screen 400 than object 411a. FIG. 4b presents on the left hand side the 2D image of the screen 400 and of the object 411a as perceived by the left eye of the observer, and on the right hand side the 2D image of the screen 400 and of the object 411b as perceived by the right eye of the observer.
In the depicted situation, the right eye thus sees less of the object 411 than the left eye. The brain picks up this disparity information from the left image and the right image, which indicates to the brain that the object 411 is closer to the observer than the screen 400. However, the virtual object 411 is cut off by the left edge of the screen 400, and thus the virtual object 411 is only partly visible. As a result, the border of the screen 400 seems to block the view onto the object 411, indicating to the brain that the screen 400 must be in front of the object 411.
This phenomenon, in which disparity information indicates that an object is in front of a screen, but in which the border of the screen seems to partly occlude the object, is referred to as reverse half-occlusion. Such a conflict between different depth indicators is a contradiction to the laws of viewing geometry that the human brain is used to experiencing in the real world, which causes binocular rivalry creating difficulties in seeing the scene clearly. Binocular rivalry may cause eye strain, nausea, disorientation and other adverse symptoms which lead to a less enjoyable viewing experience. Anomalies in the image tend to attract the observers attention to the area in an attempt for the brain to solve the discrepancy. The effect of reverse half-occlusion is also referred to as reverse occlusion or as a window violation.
3D content creators may limit the effect of reverse half-occlusions in their content.
This could be achieved for instance by limiting the forward disparity to ensure that no object in front of the screen touches the edge of the screen. The comfortable viewing space (CVS) is defined as the 3D space in front and behind the screen that virtual objects are allowed to be in and be comfortably viewed by the majority of individuals. Limiting the forward disparity thus reduces the usable 3D space to one half.
Further, content could be artificially created such that it does not cause any window violations by making objects move to the rear half of the viewing space before they enter or exit the scene. Further, an object could be made to enter or exit the scene so quickly that the brain is not able to notice the window violation. Both approaches limit the freedom of designing content, though.